[llvm] [CAS] LLVMCAS implementation (PR #68448)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> > (be great if this could be broken up a bit more - like the TrieRawHashMap looks like it'd be a fine standalone patch to introduce the data structure and its unit tests - maybe similarly with the other VirtualOutputBackends patch, the raw_ostream_proxy might be separable (& it'd be nice if the other parts of the functionality were built up more incrementally too))
> > But maybe it's just not practical - it's unfortunate when things get to this state, though I understand the value of prototyping out of tree - it makes it hard to adequately review things when they come into the project this way.
> 
> Both of the things you mentioned above are separate commits in those PRs already and I will continue to break apart more functions if needed. I am happy to put up individual PRs if that is better for reviewers while shrinking this mega PR.

Ah, fair enough. Not ideal (it's still sort of one review for the whole set of changes & any updates get disjoint (because they've got to be stacked on top of the already complex stack, rather than added into the stack back where the original change was made, etc)

Such is life - eventually we'll get some better dependent-patch review process I hope.

https://github.com/llvm/llvm-project/pull/68448